Method for the production of high-purity ethylene glycols

ABSTRACT

A method is disclosed for obtaining high-purity ethylene glycols in which the improvement consists in reducing the oxygen-containing organic impurities by treating their aqueous solutions as obtained during the processing of ethylene oxide with a solution of an alkali metal borohydride, sodium borohydride being preferred. 
     A definitely alkaline environment is preferred. Ethylene glycols can thus be obtained which are particularly suitable for use in the fibre-manufacturing industry. Such glycols have extremely low UV-absorbance values.

This is a continuation of application Ser. No. 851,296, filed Nov. 14,1977, now abandoned.

This invention relates to a method for the production of high-purityethylene glycols.

According to the conventional art, ethylene glycols are producedcommercially according to a two-stage method. The first stage comprisesthe step of producing ethylene oxide, and during this stage, thecatalytic oxidation of ethylene is carried out in the presence of air orof oxygen. The second stage consists in the hydration of the ethyleneoxide, as prepared in the previous stage, to ethylene glycol.

As a result of the oxidation of ethylene there are produced, in additionto ethylene oxide, carbon dioxide and water, also small amounts ofimpurities which contain oxygen, such as CH₃ CHO, HCHO together withother aldehydes and oxygen-containing compounds. These compounds, alongwith other compounds as formed in the purification and recovery stages,can be found, distributed among the several liquid or gaseous streams ofthe recovery and purification cycle until they are finally found both inthe as-produced ethylene oxide and in the liquors present in theinstallation.

The liquors present in the installation contain, in addition to suchby-products, also a not negligible amount of monoethylene glycol.

In the subsequent stage, that is that of glycol-production, the ethyleneoxide is admixed with water, at appropriate concentrations and pHvalues, and is then treated at a temperature of about 100° C.-200° C.,the hydration of ethylene glycol to the glycols being thus achieved. Theas-produced glycols are then separated from the water in excess andsubjected to purification steps and to the final rectification run.

Considerable advantages could be achieved, would it be possible to usefor the hydration of ethylene oxide the same watery liquors as producedin the ethylene oxide producing stage.

Such waters, if and when utilized, should not be treated in thepurification installation prior to being dumped, the water consumptionof processing water would be smaller or even annulled for the hydrationreaction and, at least, it would be possible to recover the glycol ascontained in the waters coming from the first stage, so that the finalyield would be increased by as much as from 2% to 5%.

These facts would be conducive to a considerable improvement in theproduction of glycols.

The oxygen-containing impurities, no matter if deriving from the watersof ethylene oxide portion or contained in the oxide itself are composednot only by the aldehydes indicated hereinabove but also by compoundshaving high Ultra Violet extinction coefficients and go to pollute thefinished monoethylene glycol which shows high-absorbance values of UltraViolet radiations. Such high values do not fulfil the most stringentspecifications for the use in the fibre-manufacturing industry.

According to the conventional art, many authors suggest for such processexpensive and intricate approaches such as for example the use ofactivated charcoal or of exchange resins which considerably affect thepurification cycle.

These suggestions have not been industrially tested hitherto and in thepresent-day installations which are run for the production ofhigh-purity glycols, the waters of the ethylene-oxide-production stageare dumped from the cycle, whereas, for the hydration of ethylene oxidewater is fed at a high purity rating, such as demineralized water, forthe necessary water topping up.

The present invention permits that there may be employed, for thehydration of ethylene oxide, the same waters produced in the preparationof ethylene oxide, without thereby incurring the shortcomings enumeratedhereinabove.

The present invention contemplates the step of adding borohydrides ofalkali metals to the glycol waters of the installation. Such additioncan be carried out both before and/or after the admixture of such waterswith ethylene oxide. The borohydrides of alkali metals can be used bothin the form of solids or in that of stabilized solutions.

At present, the borohydride which is the most attractive from aneconomical standpoint is sodium borohydride, NaBH₄.

Sodium borohydride in aqueous solution, however, tends to becomedecomposed into H₂ and NaBO₂, such phenomenon being fostered by atemperature increase and hindered by high pH values. At present, NaBH₄is stored either in the solid powdered state or as an aqueous solutionstabilized with caustic soda, the weight ratio of soda to borohydridebeing about 3 times. A typical commercial composition is the following:

    ______________________________________                                        NaBH.sub.4         12% by weight                                              NaOH               38% by weight                                              H.sub.2 O          50% by weight                                              ______________________________________                                    

The following TABLE reports the decomposition velocity of NaBH₄ from itssolutions.

    ______________________________________                                        SOLUTION       Temp.                                                          NaBH.sub.4                                                                          NaOH    H.sub.2 0                                                                              °C.                                                                          pH   Decomposition in time                       ______________________________________                                        0.4   3.8     balance  24    14   about 1% after 24 hrs                       0.4   3.8     balance  47    14   about 16% after 24 hrs                      12    38      balance  25    14   about 0% after 24 hrs                       0.009 0.029   balance  150   11.5 100% after 45                                                                 seconds approx.                             0.017 0.055   balance  150   11.8 100% after 60                                                                 seconds approx.                             0.009 0.029   balance  80    11.5 100% after 2                                                                  hrs. approx.                                ______________________________________                                    

A surprising aspect of the invention is that the reduction of theoxygen-containing compounds takes place levels of a few parts permillion (ppm) by employing nearly stoichiometrical amounts of NaBH₄ withrespect to the compounds to be reduced, while operating at the sametemperatures of the hydration step (40° C. to 200° C.) and under pHvalues even below 12. It is preferred that the amount of alkali metalborohydride employed be approximately 1-3 times the stoichiometricamount required for reducing the oxygen-containing compounds.

The glycol obtained by treating the glycolic waters of the installationwith NaBH₄ shows low values of UV-absorbance and fulfils the most severespecifications as prescribed for the use in the fibre-producingindustry.

In addition to the considerable advantages achieved when using thewaters of the ethylene-oxide production stage for the hydration of sameoxide to glycols as outlined above, the present invention permits otherimprovements in the overall technology of the glycol production.

The ethylene oxide to be forwarded to the production of the glycols doesnot require the final purification treatments, the result being savingsin first and running costs. In addition to that, in the rectificationunder vacuum of the monoethylene glycol lower reflux ratios can beadopted.

Inasmuch as the commercial technology for the glycol production haveseveral processing patterns, it is not possible to indicate beforehandthe points which are preferred, in the installations, for reducing thepresent invention to constructive practice.

A skilled technician is capable, at any rate, to spot the procedures forworking this invention, once he has, before him, the processing layoutof any of the commercial technological processes for glycol production.

The concept of the present invention includes the method for removingoxygen-containing compounds contained in aqueous solutions by treatingsuch solutions with alkali metal borohydrides at a pH of from 10 to 12.

A better understanding of the advantages and the procedures of thepresent invention can be achieved from the ensuing few Examples whichare anyway not to be construed as limitations.

EXAMPLE 1

A glycol-producing industrial plant is fed with high-purity ethyleneoxide (aldehyde value 20 ppm as acetaldehyde) as well as with wateryliquors consisting for the 37% of recycled waters and for the 63% ofdemineralized water. Upon hydration and separation of the excess water,monoethylene glycol is separated and rectified under vacuum.Monoethylene glycol exhibits UV absorbance values of 0.04 at 220millimicron and of 0.005 at 260 millimicron wavelength. The productfulfils the most stringent specifications for use in the production offibres.

EXAMPLE 2

A glycol-producing commercial plant is fed with high-purity ethyleneoxide (aldehyde contents 20 ppm as acetaldehyde) as well as with waterswhich are composed for their 37% by recycled waters and for their 63% bywater coming from the ethylene oxide section. Upon hydration andseparation of the excess waters, the mono-ethylene glycol is separatedand rectified under vacuum. The monoethylene glycol displays an UVabsorbance value of 0.20 at 220 millimicron and 0.40 at 260 millimicron.The yield of the monoethylene glycol over the ethylene which has beenemployed is increased by 3.3% with respect to EXAMPLE 1. The as-producedmonoethylene glycol does not fulfil the most severe specification forfibre-making.

EXAMPLE 3

The same hydration product as obtained according to the procedure ofEXAMPLE 2 is supplemented at 150° C. with a solution of sodiumborohydride at a rate of 40 mls per cubic meter of effluent. Thesolution contains 12% by weight of NaBH₄ and 38% by weight of NaOH, thebalance being water. After the operations of separation andpurification, the values of Ultra Violet absorbance of the as-producedmonoethylene glycol are 0.05 at 220 millimicron and 0.020 at 260millimicron. This monoethylene glycol fulfils the most severespecifications for use in making fibres.

EXAMPLE 4

A glycol-producing commercial installation is fed with non-purifiedethylene oxide (aldehyde contents, as acetaldehyde, definitely over 400ppm) as well as with waters composed for 37% by recycled waters and for63% by water coming from the ethylene oxide section. After hydration,separation of the excess waters and purification, the as-producedmonoethylene glycol shows UV-absorbance values of 0.40 at 220millimicron and of 0.60 at 260 millimicron. This product is unsuitablefor fibre-making. The monoethylene glycol yield with respect to theethylene used is increased by 4.2% with respect to EXAMPLE 1.

EXAMPLE 5

The reaction effluent of EXAMPLE 4 is supplemented by the solution ofsodium borohydride of EXAMPLE 3, at a rate of 90 mls per cubic meter.The UV-absorbance values of the purified monoethylene glycol are 0.12 at220 millimicron and 0.025 at 260 millimicron. This monoethylene glycolis fully suitable for fibre-making. The yield of monoethylene glycolwith respect to the ethylene employed is increased by 4.2% overEXAMPLE1.

EXAMPLE 6

When operating with the charge reported for EXAMPLE 2 above, butsupplementing the waters emerging from the ethylene oxide section withthe sodium borohydride solution of EXAMPLE 3, at 60° C., at a rate of 40mls per cubic meter, there are obtained about the same values ofUV-absorbance as in EXAMPLE 1, with savings of 40% approximately ofsodium borohydride as compared with EXAMPLE 3.

We claim:
 1. A method for the production of ethylene glycol of highpurity having low values of U.V. absorbance comprising hydration ofethylene oxide to ethylene glycol wherein the hydration of ethyleneoxide is effected using water produced during the manufacture ofethylene oxide by the catalytic oxidation of ethylene, additional watersufficient to effect hydration of the ethylene oxide, and an alkalimetal borohydride.
 2. A method as claimed in claim 1 wherein said alkalimetal borohydride is sodium borohydride.
 3. A method as claimed in claim1 wherein said hydration is carried out at a temperature of 40°-200° C.at an alkaline pH.
 4. A method claimed in claim 1 wherein said alkalimetal borohydride is employed in amounts of 1 to 3 times thestoichiometric amount required for reducing oxygen-containing compoundsin said water.